1. Technical Field
The present invention generally relates to an apparatus and method to calibrate an exhaust flow measurement apparatus, and more particularly, to an arrangement for providing a known flow reference signal to the exhaust flow measurement apparatus and measuring the response of that unit to the reference signal.
2. Discussion
Several vehicle exhaust flow measurement methods have been developed over time. One method compares an initial concentration of carbon dioxide within a vehicle""s exhaust to a diluted carbon dioxide measurement taken after outside air has been mixed with the exhaust. This method utilizes the dilution ratio of carbon dioxide to calculate the quantity of other exhaust gas constituents such as carbon monoxide.
A constant volume sample is coupled to the vehicle exhaust to collect gas samples for testing. It should be appreciated that the constant volume sampler functions properly only when the sample is completely gaseous. To assure that water does not condensate within the constant volume sampler, a large quantity of outside air must be added to the vehicle exhaust. Unfortunately, after a water correction factor is taken into account, the method is accurate to only within 10 percent.
Another exhaust flow measurement technique incorporates a smooth approach orifice and a constant volume sampler. The constant volume sampler is plumbed directly in-line with the vehicle exhaust and the smooth approach orifice. Thus, the sum of the gas flow from the vehicle exhaust and the gas flow through the smooth approach orifice equals the flow through the constant volume sampler. Accordingly, if the constant volume sampler flow rate is known and the smooth approach orifice flow rate is known, the vehicle exhaust flow may be calculated. However, only very small pressure drops are present within the smooth approach orifice. As is known, it is very difficult to accurately quantify small changes in pressure and this method is therefore subject to relatively large percentage errors.
A preferred method of exhaust flow measurement involves using an ultrasonic measurement device. Ultrasonic exhaust flow measurement units are used to quantify exhaust gas volumes directly from the tailpipe of a vehicle. Each exhaust flow unit has its unique calibration curve relating the output signal measured by the device to the amount of flow passing through it. The calibration curve is provided by the manufacturer of the unit and is built into the program that controls it.
As is known, the physical characteristics of any precision measurement device are subject to changes over time. These changes necessitate revising the calibration curve to reflect the changes in the measurement device. In the past, recalibration required sending the entire exhaust flow measurement device back to the manufacturer. This process was time consuming and very expensive.
Accordingly, it is an object of the present invention to provide a method and apparatus to calibrate an exhaust flow measurement device.
It is another object of the present invention to cost effectively and accurately provide a known flow reference signal for use in calibrating an exhaust flow measurement device.
The present invention includes a method and apparatus for calibration of an exhaust gas measurement system for a motor vehicle. The method includies the steps of providing a flow of air to the exhaust gas measurement system having a known flow rate, measuring and storing a signal output from the exhaust gas measurement system, repeating the first two steps a predetermined plurality number of times, calculating a calibration curve based on the known flow rates and the stored signal; and using the calibration curve in the exhaust gas measurement system.